1. Field of the Invention
The present invention relates to a ferrite sintered body and, in particular, to a ferrite sintered body having a high surface resistance and a low core loss. The invention also relates to a ferrite sintered body having a high magnetic permeability (μ) and a high saturation magnetic flux density (Bs) at the same time, and to a manufacturing method thereof, as well as ferrite cores and ferrite coils using these ferrite sintered bodies.
2. Description of Related Art
A ferrite sintered body, for example, is applied to a ferrite core mounted on an inductor used around a power source, and this application demands a high magnetic permeability, a high saturation magnetic flux density, a high electrical resistance, and a low core loss.
Japanese Patent Application Laid-Open No. 10-45415 discloses a ferrite sintered body composed of a ferrite material of high saturation magnetic flux density that contains, as main component, 54 to 75 mol % of Fe2O3, 10 to 30 mol % of ZnO, 10 to 25 mol % of NiO, and 3 to 10 mol % of CuO, and further contains 0.1 to 5 parts by weight of Bi2O3 and 0.1 to 5 parts by weight of MoO3, to 100 parts by weight of this main component.
Japanese Patent Application Laid-Open No. 2001-151564 discloses a ferrite sintered body that contains 48 to 50 mol % of Fe2O3, 1 to 5 mol % of CuO, and 0.1 to 1 mol % of MnO, and that further contains, as subsidiary component, 0.01 to 0.2 parts by weight of MgO, 0.05 to 0.5 parts by weight of SiO2, 0.05 to 0.5 parts by weight of Al2O3, and 0.01 to 0.2 parts by weight of Cr2O3, to 100 parts by weight of a main component in which the mole ratio of ZnO/NiO constituting the residue is 1 to 1.6.
Japanese Patent Application Laid-Open No. 2002-187769 discloses a ferrite sintered body comprising mainly Fe, the main body of which is an oxide of spinel structure containing at least one of Zn, Ni, or Cu, wherein the content of an oxide comprising mainly at least one of Zn, Ni, or Cu is less than 0.01% by volume.
In these ferrite sintered bodies, the ferrite sintered body of Japanese Patent Application Laid Open No. 10-45415 is prepared by firing at 950 to 1250° C., and the ferrite sintered bodies of Japanese Patent Applications Laid-Open Nos. 2001-151564 and 2002-187769 are prepared by firing at 950 to 1400° C. Although these publications disclose no firing conditions other than the firing temperature, generally, a ferrite sintered body is obtained by placing a forming body (green body) on a plate-shaped kiln furniture, and allowing to flow air into a firing furnace without disposing around the forming body any shielding member for shielding the flow of air around the forming body.
However, the ferrite sintered body of Japanese Patent Application Laid-Open No. 10-45415 is high in saturation magnetic flux density but low in surface resistance and high in core loss.
According to Japanese Patent Applications Laid-Open No. 2001-151564, there are attained the characteristics of: Saturation magnetic flux density Bs≧400 mT; Magnetic permeability μ≧500; and Volume resistivity value≧100 MΩ·cm. However, an obtainable ferrite sintered body is low in surface resistance and high in core loss, since the concentration of Zn existing in the vicinity of the sintered body surface is not controlled.
The ferrite sintered body of Japanese Patent Application Laid-Open No. 2002-187769 is high in both saturation magnetic flux density and magnetic permeability, but low in surface resistance and high in core loss.
These problems in the above ferrite sintered bodies may be caused by too low or too high Zn concentration in the surface vicinity of the sintered body than that in the interior.
In these ferrite sintered bodies, to increase the efficiency of firing, air exceeding 1 m3/minute per volume of 1 m3 is allowed to flow into a firing furnace at an atmospheric pressure, resulting in a great velocity of the flow of air. Therefore, a large amount of Zn evaporates from the surface of a forming body during firing, and hence the Zn concentration of the surface portion of the obtained ferrite sintered body is extremely low than that in the interior. When the Zn concentration in the sintered body interior is taken to be 1, the Zn concentration in the surface vicinity is not more than 0.5, so that there are considerable differences in the Zn concentration between the interior and the surface.
Additionally, to avoid a decrease in the Zn concentration in the surface, it is necessary to perform firing with a forming body buried in powder containing much Zn. However, the Zn concentration in the surface of the ferrite sintered body so obtained is extremely high than that in the interior. When the Zn concentration in the sintered body interior is taken to be 1, the Zn concentration in the surface vicinity is not less than 1.5, so that there are large differences in the Zn concentration between the interior and the surface.